The present invention pertains generally to a welding machine for thermoplastic film or foil welding and more particularly to a welding machine for producing connection seams on a material to be welded lying on a support plate with a folding device arranged in front of the welding site for folding over an edge zone of the material to be welded.
West German Patent No. DE-PS 839,754 discloses a plastic welding machine for producing lap joints, in which an edge strip of a lower layer of the material to be welded is folded over the edge of an upper layer of material to be welded by means of a folding device in an overlapping manner.
The folding device has an essentially helical folding sleeve narrowing in the shape of a funnel, into which the front end of the edge strip of the lower layer of the material to be welded must be pushed manually in the prefolded state. Aside from the fact that it is relatively difficult to introduce the end of the edge strip in the case of rigid materials to be welded, and as a consequence of the generally high coefficient of friction of thermoplastics, it is also necessary to ensure that the edge of the folded-over edge strip that extends at right angles to the direction of the seam is exactly flush with the corresponding edge of the layer of material to be welded which is not folded over. Otherwise a weld seam extending in parallel to the side edge cannot be produced.
The same difficulties occur when a border folded over once is formed, because the leading end of the edge strip to be folded over must be introduced into a helical folding sleeve in the same manner as in the case of overlap welding.